Jamie Goode: Vines and the science of drought resistance

One of the big challenges hitting wine regions in many parts of the world is drought. And the Western Cape knows all about this after the run of very dry years from 2015 that caused a lot of problems well beyond viticulture.

This is an area that is well used to dry summers. Typical weather patterns are for warm, dry growing seasons with enough rainfall in the winter for ground reserves and dams to be replenished in time for the next few months of dry weather. This is typically referred to as a Mediterranean climate and it is shared by many of the world’s wine regions. Here, periodic droughts are one of the key challenges in growing wine grapes, and this seems to be getting worse with climate chaos.

Most breeding work for new varieties has been with disease resistance in mind. But it seems that drought tolerance is something that could be an interesting target in the future. Growers in many parts of the world are looking to increase the resilience of their vineyards in the face of drought, and one of the ways of doing this is using more resistant plant material, both scion and rootstock. The problem here is that there are lots of factors involved in making one vineyard more drought resistant than another, and the genetics of drought resistance in vines is likely to be complicated and multigenic.

What are the factors involved? First of all, the type and depth of soil. Some soils are better at storing water. Sand and gravel soils are very free-draining and can’t hold onto water. Soils with clay or limestone have much better ability to hold onto water. Soils that allow roots to go deep in search of water reserves are clearly better for dry climates than those that keep the root growth shallow. But aside from physical properties of the soil, management decisions matter. For example, leaving a mulch layer on the soil surface will reduce evaporation and keep soil temperature down. Incorporating organic material will enhance the ability of soil to retain water. Having things other than vines growing in the soil will create a soil structure that allows better water infiltration when it does rain, and also increases soil organic material which helps the soil hold onto the water. But herein lies a challenge: where is the balance point between these benefits of other things growing in the soil, and the negative effects of competition for the vines in a very dry climate? Some care is needed here, but keeping the soil bare all year round is likely to be a bad strategy.

Then there’s the planting density, and the way the vines are grown. In hot, dry climates, bush vines seem to do much better. They have limited canopy growth, and the canopy provides dappled shade for the grapes. This reduces the demand for water. It’s remarkable to see how well unirrigated bush vines can do in very dry climates, and typically the planting density will be relatively low. Another variation on this them is the single-wire umbrella sprawl that used to be popular in California and many Australian regions. Modern viticulture is in love with the VSP canopy (vertical shoot positioning) which provides a wall of leaves and allows the fruit zone to be clear of shade. It’s great for cooler, damper climates, but not ideal for hotter, drier ones. Typically VSP systems will be irrigated, and often they are given quite a bit of water early on in the season, creating a big canopy that then requires more water for maintenance.

Irrigation is also worth discussing here. Giving vines regular, short drinks keeps the root zone near the surface, and creates an addiction for more water. Irrigating only when the vine really needs it, and then in larger bursts, helps the root system grow deeper and creates more resilience. It also limits canopy growth.

What about vine material? The mechanisms of what makes one vine drought-resistant and another drought-susceptible are poorly understood, and there are conflicting theories. The complicating factors here are management strategies and rootstock choice, which are going to have an effect whatever the variety. And then there’s vine age: older vines typically have a bigger root system, have less vigorous canopies, and may have experienced epigenetic changes adapting them to the environment. But one way we can come up with a shortlist of more drought-tolerant varieties is to look at old regions with very dry climates to see what varieties thrive there. They may not always be better than the likes of Chardonnay, Sauvignon Blanc and Cabernet Sauvignon, but they are good candidates for trials. We can also look at the varieties that seem to do well in the Western Cape when grown without irrigation.

What of the science of drought resistance? One popular concept is that of isohydric and anisohydric vines. In 2003 Hans Schultz published the results of a study looking at Grenache and Syrah vines grown in Geisenheim and near Montpellier. This is where he introduced the concept of isohydric and anisohydric plant behaviour to viticulture. It relates to the way that plants regulate their stomata. These pores in the leaf regulate transpiration, the movement of water from the roots to the aerial parts of vines. Plants want to keep their stomata open to facilitate gas exchange because without carbon dioxide from the air photosynthesis will stop. But they don’t want the leaf water potential to drop so much that their hydraulic system breaks down through embolism, caused by cavitation in the xylem, where the pressure on the column of water rising up the plant through these vessels breaks and a bubble forms.

Abscisic acid is the signal from the roots that controls stomatal conductance. As the soil water status decreases, so does leaf water potential. Isohydric plants like maize, poplar and sugar cane maintain their leaf potential at a constant level, independent of the soil water potential. So they shut their stomata to do this. Anisohydric plants are likely to keep their stomata open longer: barley and sunflower fall into this group. Schultz noted that there are differences in stomatal sensitivity during drought among cultivars and clones. Some limit transpiration to compensate for the tendency of their xylem to fail through cavitation, while others have their stomata open longer. This translates into the ability of different varieties to tolerate drought. Grenache is seen as an isohydric variety, while Syrah is a classic anisohydric variety.

This seems to make sense. In a dry climate, the vines that shut their stomata earlier will conserve water in the soil, at the expense of carrying on with photosynthesis. They are more frugal with a limited resource. But this story may be a little too simplistic. A key aspect drought resistance seems to be the plant’s hydraulic system, and the vulnerability of the xylem to embolism is important here. Stem embolism leads to failure of hydraulics and then plant death. This rarely happens in grapevines because of a phenomenon called hydraulic vulnerability segmentation, where differences in the hydraulic properties exist between stems and leaves, so that in severe drought, the leaves will be sacrificed first, protecting the stem. These distal organs are more vulnerable to xylem embolism than perennial organs so that permanent tissues are protected. The vulnerability is also lower for basal rather than apical leaves, so those are the ones that go first. It appears to be a widespread strategy among grape varieties.

Rootstocks are likely to be key to drought resistance, and choosing the right ones, as well as developing new ones are likely to be of increasing importance in a drier viticultural future. 110R and St. George are drought tolerant and can protect against cavitation events, whereas 101-14 tends to crash when the vapour pressure deficit is high. Low vigour rootstocks are likely to be less useful here than higher-vigour ones, perhaps because the latter are good at expanding deeper into the soil profile. Rootstocks are currently under-utilized as viticultural tools with just a few widely adopted. Choosing the right rootstock for dry conditions isn’t easy, though, as grapevine drought tolerance is dependent on a combination of factors: rootstock × scion (the variety grafted onto the root) × environment (soil type and climate) × management strategies (for example, training type or spacing of vines), with all these factors influencing each other. But if there’s a good place to start, then it’s probably with the rootstock itself. The way the roots spread out in the soil and their overall volume, are likely to be critical

There’s also a new theory about drought resistance. In Israel, Elyashiv Drori and colleagues have been scouting for old grape varieties with a view to finding those that might be better adapted to the conditions in the country than the foreign imports. Their search resulted in the identification of 85 different varieties, and of these they think that 60 might be interesting for wine production. They investigated this pool of varieties and one of the traits they looked at was their ability to withstand drought stress. They separated them into two groups: ‘stable’ or ‘sensitive’ to drought stress. Interestingly, one common feature of the stable varieties is that they are late blooming, and this could be the mechanism that enables their mild reaction to the initiation of drought stress. The varieties that get going earlier must sustain their leaves for around 40 days more, and this could lead to more rapid depletion of the soil water reservoir accumulated during winter. Research is on-going, but if later budburst is a feature that enables varieties to deal with dry conditions better, it’s likely to be under much simpler genetic control than the other factors identified in drought tolerance, for which we lack good genetic markers that would be helpful in breeding programs.

Hopefully, the science of drought resistance will become clearer. For now, it seems that implementing the right management strategies is key for existing vineyards, and for new ones, using varieties that thrive in drier conditions is a no-brainer, when coupled with the right drought-resistant rootstocks.

• Jamie Goode is a London-based wine writer, lecturer, wine judge and book author. With a PhD in plant biology, he worked as a science editor, before starting wineanorak.com, one of the world’s most popular wine websites.